This invention relates to the production of flat or curved sheets of thermally toughened glass, such as are used for example as motor vehicle side or rear windows.
In most countries there are official regulations specifying the fracture requirements for toughened glass sheets which are to be used as side or rear windows for motor vehicles.
Typically such regulations specify that the toughened glass sheets shall be fractured by localised impact at a defined position on the glass sheet, two particular positions being at the geometrical centre of the glass sheet and at a position adjacent the edge of the sheet. It is then required that areas of the fractured glass sheet should be selected where the particle count is a minimum and where the particle count is a maximum and limitations are placed on the minimum and maximum particle counts permissible in such areas. The minimum particle count permissible determines the maximum size of particles resulting from fracture so as to limit the danger of laceration by larger particles subsequent to fracture of the glass sheet in an accident. The maximum particle count permissible determines the minimum fineness of particles resulting from accidental fracture of the glass sheet so as to limit the danger of ingestion of fine glass particles. At present motor vehicle side and rear windows are made from glass of about 4.0 mm to 6.0 mm thickness and can be uniformly toughened so as to meet official fracture requirements.
For example glass sheets of thickness 4 mm and above meet the proposed E.E.C. standard referred to below if uniformly toughened to have a central tensile stress in the range 55 MN/m.sup.2 to 59 MN/m.sup.2. However in the interest of reducing weight there is now a trend towards the use of thinner glass to motor vehicles e.g. of about 3.0 mm thickness, glass of thickness in the range 2.5 mm to 3.5 mm being of particular interest.
In the draft standard under discussion by the European Economic Community (EEC) it is required that the number of particles in any 5 cm.times.5 cm square traced on the fractured glass, excluding a 3 cm wide band around the edge of the glass sheet and a circular area of 7.5 cm radius around the point from which fracture is initiated, should be 50 at the minimum and 300 at the maximum.
The proposed E.E.C. standard also has the requirement that the fractured glass sheet shall not contain any elongated particles with jagged ends of more than 6 cm in length, such particles being referred to as "splines".
British Standard No. BS 5282 entitled "ROAD VEHICLE SAFETY GLASS" is less restrictive than the proposed E.E.C. standard in that it specifies for glass less than 4 mm in thickness a minimum particle count of 40 in a 5 cm.times.5 cm square may be permitted and the maximum permitted particle count in a 5 cm.times.5 cm square may be 400. The British Standard also basically prohibits the presence of splines of more than 6 cm in length in the fractured test glass. The British Standard also requires that no splines are to be present in the fractured glass sheet.
It had been found difficult to toughen thinner glass sheets to meet the official fracture requirements, this difficulty being particularly evident in a size greater than about 1100 mm.times.500 mm this is about the size of the smallest vehicle rear window in current production. Many vehicle side windows are also of about this size or greater.
In U.S Pat. No. 4,128,690 assigned to the same assignee as the present application there is described and claimed a glass sheet for use as a side or rear window for a motor vehicle and of thickness in the range 2.5 mm to 3.5 mm having characteristics which have been discovered enable the sheet to meet at least the official fracture requirements laid down in British Standard No. BS 5282, the glass sheet being differentially quenched to produce in the glass sheet a distribution of regions of more highly toughened glass interspersed with regions of lesser toughened glass, the average central tensile stress in the glass being in the range of from a maximum of 62 MN/m.sup.2 for all glass thicknesses from 2.5 mm to 3.5 mm to a minimum of 56.5 MN/m.sup.2 for 2.5 mm thick glass varying inversely with thickness down to a minimum of 53 MN/m.sup.2 for 3.5 mm thick glass, and there being a distribution of medium toughened glass areas interspersed among said more highly toughened and said lesser toughened regions in the glass sheet and in which there are major and minor principal stresses acting in the plane of the glass sheet, the difference between said major and minor principal stresses in at least some of said medium toughened areas being at a maximum in the range 8 MN/m.sup.2 to 25 MN/m.sup.2, the major principal stresses in adjacent ones of such medium toughened areas being in substantially different directions, and the distance between the centres of such adjacent areas being in the range 15 mm to 30 mm, the number and location of such adjacent areas being sufficient that upon fracture there is an absence of splines of more than 6 cm in length in the glass sheet.
A glass sheet having the above characteristics was produced, as described in U.S. Pat. No. 4,128,690, by quenching a distribution of regions of the glass sheet at a maximum rate so that interspersed regions of the glass sheet are simultaneously quenched at a minimum rate, regulating said maximum quenching rate and the size and spacing of the regions of the glass sheet which are quenched at a maximum rate to give the above characteristics.
In carrying out the above method quenching was effected by directing quenching jets at the glass sheet, and imparting a vertical oscillation or a circular oscillation to the quenching jets to produce the required distribution of regions of the glass sheet quenched at a maximum rate. The quenching could also be effected by directing stationary quenching jets at the glass sheet to produce the required distribution of regions of the glass sheet quenched at a maximum rate.